1. Field of the Invention
The present invention relates generally to the generation of power from wind generators and more particularly to a novel control system for utilizing the power generated by a wind generator.
2. Description of the Prior Art
A theoretical and empirical analysis of windmills reveals that the power output, P, of the windmill is directly proportional to the radius, R, of the blades, the speed, V, of the wind, the mass density, .rho., of the air in the wind and a power coefficient, P.sub.c, according to the relationship EQU P.alpha.P.sub.c .rho.R.sup.2 V.sup.2.
The power coefficient, P.sub.c, is also a function of wind speed. More particularly, it is a function of the geometrical arrangement of the windmill and of the tip speed ratio 2.pi.NR/V where N is the angular velocity of the blades, and R and V are as previously defined.
The power coefficient, P.sub.c, has been determined in wind tunnel tests for various blade arrangements. From the foregoing analysis in the tests referred to, it is found that for a given windmill there is an optimum angular velocity of the blades at which maximum power is obtained.
A further characteristic of winds which is of interest is the distribution of the wind velocities over a period of time.
Winds are ordinarily classified in two groups, prevalent winds and energy winds. In wind studies, it is found that prevalent winds blow about 75 percent of the time and energy winds blow about 25 percent of the time. Energy winds however have average velocities about 2.3 times those of prevalent winds and therefore contain about 75 percent of the total energy generated by winds.
Since, theoretically a maximum of only 59.3 percent of the power in the wind is obtainable by a windmill to do work and that in practice only 30-40 percent is actually obtained using the most efficient windmills presently known, it is extremely desirable to not only have a more efficient system but one which is efficient over a wide range of wind velocities. Yet, it is known that conventional windmills employed in the generation of electrical energy typically operate only at relatively low wind velocities and therefore do not take advantage of the energy available at higher wind velocities.
For achieving maximum efficiency of conversion, commercial wind generators are desired to operate at a fixed wind speed to rotor tip speed ratio. Because of the random nature of the wind, an a.c. generator driven by wind powered rotor will deliver electricity with variable voltage and frequency between the cut-in and rated speed range of the wind power system. The variable frequency power if supplied to a motor driven appliance will cause the appliance to operate at irregular speeds.
Further, to maintain the cost per kilowatt hour of energy produced as low as possible, it is essential to utilize fully the output from the wind generator. That is, the generated power between cut-in and rated speed should be utilized by matching the load to the generator's momentary capacity. This load matching involves following the generator's output versus wind speed characteristics over its operating range by an infinitely variable load. Of course, it is impossible to obtain an infinitely variable load, so in practice, the available load may be divided into a series of small units with each load switched in and out of the circuit to match the generator's instantaneous output.